Inkjet printing technology is used in a wide variety of image forming apparatuses for household use and industrial use.
Home-use inkjet printers, which generally use a water-based ink, cannot achieve adequate image quality unless the recording medium used has high ink absorption property. On the other hand, industrial-use inkjet printers, using a solvent with rapid drying property and with high ability to penetrate recording media, perform printing on the types of recording medium on, which images cannot be printed with water-based inks. In the latter case, however, the solvent used evaporates from the water-based ink, which causes problems of bad smell, dangerousness, and toxicity of the solvent. Though inks using less volatile solvents are available, however, the inks dry very slowly, causing problems of poor image quality and poor fixing property. Also available are hot melt inks, which are solid at room temperature, however, these are waxy inks, still causing problems of weak surface strength of fixed images and of wide variations in fixing property of the inks on different types of recording media.
In contrast to these inks, ultraviolet curable inkjet recording inks, which use volatile solvents and smell less, are capable of forming images even on recording media without having ink absorption property and of increasing the process speed of forming images by shortening the fixing time. However, when a recording medium is not sufficiently wet with an ultraviolet curable inkjet ink, the ultraviolet curable inkjet ink does not spread over a surface of the recording medium, resulting in inadequate image quality and in degradation of fixing property due to the reduced contact area between the recording medium and the ink. Furthermore, when the ink layer thickness increases after printing with an ultraviolet curable inkjet ink, the amount of ultraviolet light reaching inside of the deposited ink layer becomes excessively small, causing curing failure of the deposited ink layer.
In order to solve the above problems, an ultraviolet curable inkjet recording ink containing a fluorine surfactant which is capable of remarkably lowering the surface tension of the ultraviolet curable inkjet recording ink has been developed (Patent Literature 1; Patent Literature 2).
However, since such fluorine surfactant does not possess the property of curing on exposure to ultraviolet light, bleeding of such fluorine surfactant takes place from a deposited layer of the ultraviolet curable inkjet recording ink after the deposited layer has been cured.
In order to solve this problem, it is proposed to use fluoroalkyl acrylate or silicone acrylate in ultraviolet curable inkjet recording inks as a wetting enhancer which reacts with ultraviolet light to cure (Patent Literature 3; Patent Literature 4). Although bleeding of such wetting enhancers can be avoided by their ability to react with ultraviolet light to cure, such reactive wetting enhancers are inferior in improving the wettability to surfactants. When such reactive wetting enhancer added into an ultraviolet curable inkjet recording ink is greatly increased in amount to improve the wettability, a considerable amount of the reactive wetting enhancer molecules remain unreacted after the curing reaction, causing bleeding of the unreacted reactive wetting enhancer residue. The amount of the unreacted reactive wetting enhancer residue can be reduced by increasing the amount of a polymerization initiator or irradiated light. However, increasing the amount of the polymerization initiator may adversely influence the physical properties of the coated membrane after curing reaction and increase the toxicity of the ink before curing. Furthermore, in order to increase the amount of irradiated light, it is necessary to provide additional expensive ultraviolet lamps, which is undesirable in terms of cost and electric power consumption.